roachtest: set config.Quiet to true

After refactoring in [1], the default of config.Quiet
was set to false since the roachprod CLI option is intended to
set it to true. This resulted in an unwanted side-effect, namely
roachtests running with the new default. Consequently, test_runner's
log ended up with a bunch of (terminal) escape codes due to (status)
spinner.

This change ensures roachtest explicitly sets config.Quiet to true.

[1] cockroachdb#99133

Epic: none

Release note: None

pkg/cmd/roachtest/main.go

@@ -353,6 +353,8 @@ runner itself.
 		fmt.Fprintf(os.Stderr, "unable to lookup current user: %s\n", err)
 		os.Exit(1)
 	}
+	// Disable spinners and other fancy status messages since all IO is non-interactive.
+	config.Quiet = true
 
 	if err := roachprod.InitDirs(); err != nil {
 		fmt.Fprintf(os.Stderr, "%s\n", err)